Reperfusion injury is an important clinical disorder which may become manifest during blood reperfusion of ischemic tissues. It is believed that reperfusion injury may cause significant damage to the myocardium following thrombolytic treatment of acute myocardial infarction, after emergency coronary artery bypass surgery, during blood reperfusion after routine bypass surgery, and in the transplanted heart which has been reperfused. The goals os this proposal are to develop the laboratory expertise, theoretical concepts, and experimental models necessary for the study of reperfusion injury in the microcirculation. Although many investigators have studied reperfusion injury, few have done so by direct observation in the microcirculation. The use of intravital microscopy will enable visualization of the rheological aspects of reperfusion injury in site, and thus aid in the understanding of this disorder. The proposal describes three phases of training. In the first phase, the examination of the effects o cardioplegia solutions on the microcirculation will be completed. Changes in the rheologies of intravascular leukocytes and microbubbles will be qualified, and evidence of reperfusion injury will be characterized. It is well known that changes in the endothelial surface, and thus its interactions with the blood, are essential in the development of reperfusion injury. Therefore, in the second phase of this project, the basic physiology of the endothelial surface will be examined. Specifically, experiments will be designed to evaluate what effect the interaction of the endothelial glycocalyx with plasma proteins has on capillary hematocrit. A fundamental knowledge of these interactions under controlled conditions with a relatively simple blood cell (the red cell) will provide the necessary background for further studies of the pathogenesis of reperfusion injury in which changes in the endothelial surface are important. The third phase of the proposal will entail the systematic investigation of reperfusion injury. First, leukocyte activation will be studied in vitro. Then, a neutropenic hamster model will be developed to determine if reperfusion injury can occur in the absence of leukocytes. Finally, the role of leukocyte-endothelial interactions during reperfusion injury will be examined in vivo. Two modulators of leukocyte adhesion, adenosine and monoclonal antibodies to the leukocyte adhesion complex, will be used in these studies.